This invention relates to copper oxide-based ceramic superconducting materials, and in particular to a process for the production of large yields of such materials in the form of highly oriented platelets exhibiting a superior ac magnetic susceptibility when compared to powders.
The recently discovered copper oxide-based superconducting materials have great potential for use in such applications as energy production, electronics, transportation, medicine, and research. Material parameters for such technologies require a high and stable current density (J.sub.c) on the order of 10.sup.6 A/cm.sup.2 or greater at 77K. Although J.sub.c 's greater than 10.sup.6 A/cm.sup.2 at 77K have been achieved in thin films, limited success has been achieved with sintered polycrystalline samples. The limitations are related to the intrinsic properties of the ceramic superconducting materials.
The existence of an anisotropic conduction plane in the crystal structure of these materials, coupled with the existence of grain boundaries formed during powder consolidation limit the current densities achievable with bulk processed materials. In any one particle there may exist several unaligned grains causing the conduction planes to be misaligned. Recent theories explaining the concept of superconductivity indicate that misalignment of conduction planes is one of the factors which leads to loss in energy, thus decreasing the current density of the material. Also, in bulk superconductors, grain boundaries act as reservoirs for impurities and other multiphase components, resulting in further energy loss. Therefore, the homogeneity of the synthesized powder has been found to be highly important to the achievement of high current densities in this material.
The orientation of single phase superconducting oxide particles would allow conduction to be more continuous within each particle. Due to the crystal structure of, for example, YBa.sub.2 Cu.sub.3 O.sub.x, the ideal morphology of these particles is plate-like with a short c-axis and conduction occurring in the relatively large, flat a-b plane. The use of materials in the form of such platelike crystallites would allow for improved grain alignment in subsequent forming processes, This grain alignment will further increase the critical current density.
Several researchers have indicated methods of growing single crystals. However, these methods utilize fluxes (e.g., excess CuO) and the formation of a total melt at temperatures greater than, for example, 1020.degree. C. for YBa.sub.2 Cu.sub.3 O.sub.x, which is undesirable. YBa.sub.2 Cu.sub.3 O.sub.x melts incongruently in air at 1020.degree. C. On cooling, such incongruently melted materials form eutectic phases which are insulating or semiconducting. The single crystals resulting from complete melt techniques comprise only a small portion of the material, and reside in the cracks of the solidified product, making separation of the crystals from the melt extremely difficult and costly.